Were an axion-like particle (ALP) to be ever discovered, it would be
compelling to explore whether it has something
to do with the strong CP problem [1]}, [2]}, [3]}, [4]}, and/or whether it can be a viable dark matter
(DM) candidate [5]}, [6]}, [7]}.
The canonical QCD axion (aka “invisible axion”) satisfies
\(m_a^{\rm QCD} f_a = m_\pi \, f_\pi \,\frac{\sqrt{z}}{(1+z)}\,, \qquad \text{where}\quad z=\frac{m_u}{m_d}\,\) 
